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Fireball from the deepfreeze

By Gabrielle Walker

IF COMETS are harbingers of great events, there should be momentous times ahead. Take the dramatic fireworks in July 1994 as Comet Shoemaker-Levy plunged into Jupiter, leaving vast bruises on the giant planet. Or Comet Hyakutake, which in March this year came so close to the Earth that it was visible even through the glare of city lights.

But the best may still be to come. Next spring, if predictions hold true, the northern skies will be lit by a brilliant new comet, which will arc through the sky trailing a rich tail and outshining every star. Comet Hale-Bopp could be the astronomical sight of the century.

Let’s keep our fingers crossed. Back in 1973 astronomers predicted great things from Comet Kohoutek, only to find that it fizzled out under the eyes of an expectant public. Their chagrin was such that they underplayed their expectations for Comet West in 1976, with the result that many people missed what turned out to be a splendid sight.

Dust balls

So why are astronomers so confident now about Hale-Bopp? One hopeful sign is that it is already astonishingly bright, even while it is still far from the Sun. Comets are balls of dust, rocks and ice. They “switch on” when they are close enough to the Sun for its warming rays to vaporise the ice and form a cloud of gas and dust, or “coma”, around the frozen nucleus. This coma scatters sunlight back to us-the more dust and gas, the brighter the comet is.

Most comets are detected when they are close enough to the Sun for their water …